(CNN) --Get it through your once-thick skull. Scientists say the bulky craniums of the human ancestor, homo erectus, may have helped the species survive some aggressive mating rituals.

After studying fossils in a region called Dragon Bone Hill in China, anthropologist Russell Ciochon of the University of Iowa concluded males of the species were clubbing one another over the head, probably to win females.

Those with thicker skulls who survived these bloody confrontations would pass that trait to offspring, Ciochon said.

"The evidence shows there may have been ritualized violence taking place," he said.

For more than 100 years, researchers have pondered the odd shape of homo erectus' skull, which looks something like a bicycle helmet.

Designed to protect the brain, eyes and ears from impact, homo erectus' head was bulkier than those of hominids before it, and after it.

In their book "Dragon Bone Hill, an Ice Age Saga of Homo Erectus," Ciochon and anatomy professor Noel Boaz say they would "lay bets that, as in many other species, we are detecting the results of sexual selection."

In the rugged world of homo erectus, a tough skull served them well, scientists said. It would have been a tremendous advantage for survival, less likely to break on impact and geared toward winning fights.

With no language, only a shaky knowledge of fire, and simple stone tools, homo erectus managed to survive for more than 1.5 million years.

Over the years other explanations for the thick skulls didn't completely add up.

For example, the skull could not save them from a major predator of the day, the giant hyena. Hyenas would likely have aimed for softer, more vulnerable parts of the homo erectus body, researchers say. And the animal's fierce teeth could pierce even the thickest pre-human head.

The researchers used modern tools to reach their conclusions, studying fossil casts, fossil bones, ethnographic records and comparative anatomy. They experimented with human cadavers to establish how much force it would take to break bones.

They gathered evidence from dozens of fossils with healed skull fractures: presumably from the hard-heads who got the girls.

Researchers compared the mating rituals of the hominids to those of bighorn sheep, who charge at each other at speeds up to 20 mph and butt heads for the right to mate with females.

So why didn't modern humans inherit this thick skull? Ciochon said evolution eventually favored a lighter skull to accommodate a heavier and larger brain. A thinner skull also would help cool the brain.

An excerpt of the book is in the February issue of Natural History magazine.